It may be noticed, for example, that milk is absent from the sacred offerings amongst the Hebrews, and this was ascribed by the late Professor Robertson Smith to the fact that all ferments were excluded from presentation at the altar,[2] it being recognised that, owing to the hot climate, milk of all kinds became rapidly sour, and in this way came to be looked upon as only fit for consumption when in that condition. It has been suggested that the prohibition referred to is on the same level as the prohibition of the use of blood, "as milk has sometimes been regarded as a kind of equivalent for blood, and containing the sacred life."[3] To this day the wandering tribes of Arabia consider the milk of their camels and flocks more refreshing when it has been slightly fermented or soured by being poured into a milk-skin on the inside of which are still sticking sour clots from the previous milking, and there shaken for a brief period; but this slightly soured milk (the _Oxygala_ of Pliny) is known widely in the East simply as leben (milk). The name is also applied to what we term buttermilk.[4] [Illustration: KABYLES SOURING MILK In the north of Africa the use of soured milk is common, and the illustration shows Kabyles shaking a skin full of milk so as to sour it. The skin has previously been used for the same operation, and, as a consequence, clots of milk are left from the previous day's use, and thus fermentation is set up.] The use of milk-skins for the carrying of milk is not confined to one country, as, while it is common all over the north of Africa, it is also known in the Pyrenees and in some parts of the Balkan Peninsula, the object being identical in each case; and when it is intended to make butter from the milk, the skin is simply rocked between the knees until the butter separates, a process of butter-making which was also used after the introduction of earthenware churns.[5] Dried soured milk is also used by the Arabs, and it is reconstituted when required by rubbing it up with a little water, and it is known as _Meeresy_.[6] The ordinary soured milk is the common article of diet, and is looked upon as being necessary at every meal, and travellers frequently refer to the use of this product, as a few references will show. Charles G. Addison states: "A supper was brought in on a round tray. In the centre was a huge pilaff of rice, and around it several small dishes of stewed meats, grilled bones, sour clotted milk called _yaoort_,[7] bits of meat roasted, etc....